1. Field of the Invention
This invention relates to an image forming apparatus for use in a reproductive device, such as a copying machine, a printer, a plotter, or a facsimile machine.
2. Description of Related Art
Known image forming apparatus in which an image is formed use a toner-flow control means having plural opening portions (hereinafter referred to as "apertures"). In such an image forming apparatus, a voltage according to image data is applied to the toner-flow control means to control toner particles to selectively pass through the apertures. The toner particles that have passed through the apertures of the toner-flow control means are deposited on a supporter (image receiving medium) to form an image. Such an image forming apparatus as described above is disclosed in U.S. Pat. No. 3,689,935, for example.
This image forming apparatus includes an aperture electrode unit serving as the toner-flow control means, and further includes a potential supply means, a toner supply means and a positioning means.
The aperture electrode unit comprises a flat plate formed of insulating material, a reference electrode, plural control electrodes and plural apertures. The reference electrode is continuously formed on one surface of the flat plate, and the control electrodes are formed on the other surface of the flat plate to be electrically insulated from one another. The apertures are formed in a row in correspondence with the respective control electrode to penetrate through the flat plate, the reference electrode and the control electrodes.
The potential supply means serves to selectively apply a potential across the reference electrode and the control electrodes of the aperture electrode unit on the basis of image data. The toner supply means serves to supply charged toner particles to the lower side of the aperture electrode unit so that the flow of the toner particles passing through the apertures is modulated according to the potential applied to the aperture electrode unit. The positioning means serves to feed and position a supporter (on which an image will be formed) in a flow path of the toner particles to be movable relatively to the aperture electrode unit.
Further, in U.S. Pat. No. 4,743,926, U.S. Pat. No. 4,755,837, U.S. Pat. No. 4,780,733, and U.S. Pat. No. 4,814,796, a first type of image forming apparatus is disclosed in which the reference electrode of the aperture electrode unit is disposed to face the toner supply means side and the control electrodes of the aperture electrode unit are disposed to face the supporter side on which an image will be formed.
On the other hand, U.S. Pat. No. 4,912,489 discloses a second type of image forming apparatus in which the control electrodes of the aperture electrode are disposed to face the toner supply means side and the reference electrode of the aperture electrode is disposed to face the supporter side. As described in the U.S. Pat. No. 4,912,489, with this construction, the second type of image forming apparatus can reduce a voltage to be applied to the control electrodes at an off-time to about a quarter of that of the first type of image forming apparatus. The term "off-time" means a time when no toner particle is deposited on the supporter, that is, a time when a blank portion of an image is formed on the supporter. On the other hand, "on-time" means a time when a toner image is formed on the supporter.
However, the first and second types of image forming apparatus have a disadvantage that the toner is liable to pass through the apertures and be attached onto the supporter even at the off-time when no toner image is formed on the supporter. Thus, image quality is unstable.
To overcome the above disadvantage, a method has been proposed of reducing a driving voltage for electric-field control of the toner by making the insulating flat plate of the aperture electrode unit thin. In this method, however, the motion of insufficiently-charged toner supplied by the toner supply means cannot be sufficiently controlled because a voltage for controlling the passage of the toner through the apertures is lowered. That is, the insufficiently-charged toner cannot be controlled by the electric field generated with a low voltage. Accordingly, it is highly possible that the insufficiently-charged toner may pass (jump out) through the apertures provided on the thin flat plate due to only a mechanical force. As a result, like the first and second types of image forming apparatus, there occurs a problem that a small amount of toner passes through the apertures even at the off-time when no toner image is formed and is deposited on the supporter. To suppress the mechanical passing (jump-out) of the toner as described above, the insulating flat plate must be made thick. However, if the flat plate is merely made thick, a dielectric gap would be larger, and the intensity of the electric field due to the control voltage would not be sufficiently increased. So, an image will be formed with low density, and high-speed recording cannot be performed.